Method of making a reversible gel drug delivery vehicle

ABSTRACT

A method for producing a drug system which is a reversible gelled matrix with entrapped drug. The gel is made by combining a polysaccharide, such as dextran, a polymer of glucose, with a natural macromolecule, such as the lectin, concanavalin A, with binding sites for monomers of the polysaccharide. The drug is released in response to exposure of the matrix to free carbohydrate containing the monomer of the polysaccharide for which the natural macromolecule binds.

[0001] This invention concerns a method for producing a drug system for delivering a drug or other agent for the treatment of a condition in a controlled release manner.

[0002] Most known ‘controlled-release’ drug systems are constant slow-release mechanisms (U.S. Pat. No. 4,145,410). These systems do not react to any inherent substances in the body to release the drug appropriately in response to that substance, ie. there is no feedback mechanism. Feedback mechanisms have been suggested to be of interest in vivo to maintain general homeostasis.

[0003] U.S. Pat. No. 4,348,387 discloses a feedback controlled insulin delivery system wherein glucose-insulin conjugates are displaced from glucose binding sites on a binding molecule by free glucose. The conjugated insulin retains its biological activity once released. However, it is not known whether other agents or drugs can be so conjugated or whether such a conjugated form of any drug or other agent would be effective.

[0004] The present invention provides a controlled drug delivery system for the controlled release of an unconjugated drug or other agent.

[0005] According to the present invention there is provided a method for producing a drug system for delivering a drug for the treatment of a condition, comprising immobilising a drug or other agent in a bio-compatible matrix containing at least one receptor for a physiological substance which will be in the environment of the matrix when administered, said receptor being activated in response to the levels of said physiological substance to effect a conformational change in the matrix allowing mobilisation and release of the drug into the environment.

[0006] The agent may comprise a naturally occurring biological agent, for example, a hormone, which may, of course, be insulin, as in U.S. Pat. No. 4,348,387, but now in unconjugated form. Because there is now no need for conjugation, other hormones or drugs which it may not be possible to conjugate or which will not work in the system of U.S. Pat. No. 4,348,387 even if they can be conjugated, or which may be physiologically ineffective or less effective or even harmful when conjugated, may be used according to the invention.

[0007] The receptor may be a binding macromolecule, for example, a lectin, which may be concanavalin A, a lectin produced from the jack bean.

[0008] The receptor may reversibly bind the physiological substance, which may be a carbohydrate, preferably glucose, or a carbohydrate containing glucose moieties, such as dextran.

[0009] Insulin, or any other agent or drug, may be immobilised in the matrix when terminal glucose molecules on dextran bind to concanavalin A to form a gel.

[0010] The conformational change in the matrix may be brought about by the displacement of terminal dextran-glucose molecules from the receptor by free glucose in the physiological environment which may of course be blood or other tissue fluid.

[0011] The conformational change may be an ungelling of the matrix allowing mobilisation and release of the insulin or other agent or drug.

[0012] The invention will be further apparent from the following description with reference to the several figures of the accompanying drawings, which show, by way of example only, one form of the system embodying same.

[0013] Of the drawings:

[0014]FIG. 1 shows a diagrammatical representation of a concanavalin-A molecule binding glucose;

[0015]FIG. 2 shows a diagramatical representation of the delivery system, in an inactive form;

[0016]FIG. 3 shows a diagramatical representation of the delivery system, in an active form;

[0017]FIG. 4 shows the drug system of FIG. 2 enclosed in a bio-compatible lattice; and

[0018] FIGS. 5 to 7 show in vitro experimental results of the drug system.

[0019] Referring now to the drawings, it will be seen that the drug delivery system is based on a concanavalin-A molecule 11. Each concanavalin-A molecule 11 has four binding sites 12 specific for sugars, with a high affinity for glucose 13 (FIG. 1). The binding of glucose 13 to the binding site 12 is, however, reversible.

[0020] The inactive form of the drug system comprises (FIG. 2) glucose present as terminal glucose moieties 15 of the carbohydrate dextran. Dextran is a branched chain polysaccharide with many terminal glucose moieties which can bind to the binding sites 12 of concanavalin-A molecules 11 and in so doing cross-link the concanavalin-A molecules together to form a viscous gel matrix. Insulin 16 is premixed with dextran gel, so that when concanavalin-A is added to the mixture, a gel is formed between the dextran and the concanavalin-A with the insulin relatively immobilised inside the gel matrix unable to escape rapidly (FIG. 2).

[0021] The binding of dextran to concanavalin-A is reversed (FIG. 3) when there is an increase in the concentration of free glucose 17 in the physiological environment. The free glucose displaces the terminal dextran-glucose 15 from the binding sites of the concanavalin A molecules. The matrix undergoes a conformational change allowing mobilisation and release of the insulin into the environment.

[0022] Once the level of free glucose falls, in response to the action of the released insulin, the displaced terminal dextran-glucose molecules will re-bind to the concanavalin-A molecules and the matrix will re-gel, thus again restricting insulin remaining in the matrix.

[0023] The mechanism of this drug system is thus repeatable and releases insulin in response to a number of free glucose insults, in a similar manner to the in vivo feedback mechanism of the pancreatic cells.

[0024] Experiments carried out in vitro show release of a drug, nitrofurantoin, from the drug system in response to a glucose load. The drug nitrofuantoin was chosen to exemplify the invention mainly because it has similar physicochemical properties to some oral hypoglyceamic drugs but is easier to assay. In these experiments the drug system comprised a mixture of the following: Dextran (from Sephadex G-25, fine beads)  99 mg Drug (Nitrofurantoin)  39 mg Concanavalin-A preparation comprising sodium chloride  15 mg lectin  3 mg Ethocel 50cp (water dispersible) 153 mg TOTAL 300 mg

[0025] The drug system above was separately tested in both sucrose containing and plain media. The sugar was present at 2% w/v. The pH was 6.2 and the media contained trace quantities of manganese and calcium, important for glucose moiety binding to lectin.

[0026] Dextran (uncross-linked AW 2-40×10 ⁶ daltons) may be used in place of sephadex G-25 fine beads.

[0027] The results are shown in FIG. 5. As can be seen from the graph, the drug was released more quickly from the lectin in the presence of sucrose. A further control experiment was carried out in which the drug system was mixed without any lectin. The results are shown in FIG. 6. A reproducible depression of drug release was seen in the presence of sucrose, probably due to to the viscosity of the diffusion boundary layers. When a correction was made for this effect in the experiment of FIG. 5, the increased rate of release of the drug was even more pronounced (FIG. 7).

[0028] The system could be implanted inside a diabetic patient with up to several months supply of insulin. However, since concanavalin-A is itself toxic, it would be desirable to enclose the matrix in a non-erodible material which allows the diffusion of free glucose and insulin into and out of the matrix, but does not allow the escape of the concanavalin-A molecules.

[0029] An example of such a material is an acrylic permanent gel, covalently cross-linked in situ to form a three dimensional grid structure 18 (FIG. 4). In addition to preventing the escape of toxic concanavalin-A into the bloodstream of a patient, it also allows an implant to be recoverable and possibly ‘recharged’ with insulin and reused. The acrylic network will also help to prevent overload of the receptor by a high level of irrelevent sugar with subsequent loss of specificity as well as preventing dose-dumping. In such a matrix, restriction of drug diffusion may occur by either an increase in viscosity or precipitation via the dextran-lectin reaction.

[0030] Alternatively, the dextran perimeter may be cross-linked with a specific agent, for example, epichlorohydrin to form a membrane of cross-linked dextran around the gel.

[0031] It will be appreciated that it is not intended to limit the invention to the above example only, many variations, such as might. readily occur to one skilled in the art, being possible, without departing from the scope thereof.

[0032] For example, when adminstering anything other than insulin or a drug which is intended to control blood sugar, the drug or agent may be released not by a feed-back mechanism but by a controlled administration of sugar, bringing literally to life the notion of a spoonful of sugar making the medicine go down.

[0033] The receptor may be any molecule, natural or engineered, having binding sites for complex branched substrates which would form a gel on binding, but would be displaceable by the ‘free’ substrate which may be naturally or artificially induced to effect release of a drug or other agent. Lectins other than concanavalin-A, which may be non-toxic or less toxic, may be used, for example, the pea (Pisium sativum) lectin. 

1. A method for producing a drug system for delivering a drug for the treatment of a condition, comprising producing a bio-compatible matrix containing an immobilised drug and at least one receptor for a physiological substance in the environment of the matrix, said receptor being activated in response to the levels of said physiological substance to effect of a conformational change in the matrix allowing mobilisation and release of the drug into the environment.
 2. A method according to claim 1, wherein the drug comprises a naturally occuring biological agent.
 3. A method according to claim 2, wherein the biological agent is hormone.
 4. A method according to claim 3, wherein the hormone is insulin.
 5. A method according to any preceeding claim, wherein the receptor is binding macromolecule.
 6. A method according to claim 5, wherein the binding macromolecule is lectin.
 7. A method according to claim 6, wherein the lectin is concanavalin A, a jack bean lectin.
 8. A method according to claim 6, wherein the lectin is a pea lectin.
 9. A method according to any one of claims 5 to 8, wherein the receptor binds the physiological substance.
 10. A method according to any preceeding claim, wherein the physiological substance is a carbohydrate.
 11. A method according to claim 10, wherein the carbohydrate is dextran.
 12. A method according to claim 9, wherein the carbohydrate is glucose.
 13. A method according to claim 10, wherein terminal glucose molecules on the dextran bind to the receptor.
 14. A method according to any preceeding claim, wherein the drug is immobised in the matrix when terminal glucose molecules on dextran bind to concanavalin A to form a gel.
 15. A method according to any preceeding claim, wherein the conformational change in the matrix is brought about by the receptor binding the physiological substance.
 16. A method according to claim 15, wherein the conformational change is an ungelling of the matrix. 